Evidence for a delayed response of riverine phosphorus exports from increasing agricultural catchment pressures in the Lough Neagh catchment

Abstract

Total phosphorus (TP) exports from the rivers draining into Lough Neagh increased between 1974 and 2000 despite lower TP inputs to the rivers from point sources. Over this period annual diffuse exports of dissolved reactive P (DRP) increased by 238%, from 17 to 57 kg P km−2 with an annual rate increase of 1.6 ± 0.3 kg P km−2 yr−1. Dissolved unreactive P exports increased by 0.4 ± 0.2 kg P km−2 yr−1, but particulate P exports did not increase. Annual exports of these three P fractions were positively correlated with annual runoff. The catchment of Lough Neagh has experienced an intensification of agriculture, with phosphorus inputs of manures and fertilizers to agricultural land increasing from 1,176 kg P km−2 yr−1 in 1925 to 3,823 kg P km−2 yr−1 in 2000. However 85% of this increase predated 1975. Increasing DRP exports after 1974 were better correlated with increasing soil P, and 70% of the increase in P accumulation by soils since 1925 occurred after 1974. Land use remained stable after 1974 with a historically low component of arable (>5%) and a dominance of grassland (<63%). The principal decade of intensification was in the 1940s, but this coincided with a greater arable component of land use. This period predated the increase in diffuse DRP exports. Intensively managed grasslands appear to be vulnerable to high DRP exports, reflecting surface application of manures, accumulation of soil P at the surface, and the creation of bypass flow pathways that facilitate the loss of P from soil to water.